While this is abhorrent, I have to say in general Firefox is just painfully slow on mobile compared to other browsers. Don't know what it's doing but I stopped using it because it's noticeably slow.
I have a pretty fast phone, and I dont notice too many problems in slow downs, what I do notice is how much more battery it uses vs chrome, I have a sneaking suspicion that google has hardware level tricks that help efficiency or they are hiding their true power usage.
I'm using Fold 5 and can tell a difference in scrolling performance between Samsung Internet and Firefox. There are also some issues regarding how the UI handles transition between folded to unfolded and vice versa.